A nuclear power plant generally has numerous valves installed therein, and, in many cases, an electric motor-operated valve drive is used for the drive of those valves. Besides such electric motor-operated drive system, valve drive systems include a hand-operated one, an air motor-operated one, hydraulic motor-operated one, etc. FIG. 3 shows a representative example of those valve drives, illustrating a section of a principal part of a certain valve drive device with an electric motor-operated valve drive attached to a slice valve thereof. The electric motor-operated valve system is configured such that a housing is fixed to a flange at a distal end of a yoke part of the slice valve so that the valve may be opened and closed by the reciprocal movement of a valve stem, by rotation of a motor.
FIG. 4 is an enlarged view of the principal part of the electric motor-operated valve drive device of FIG. 3. A worm is rotated by a motor attached to the housing. Rotation of this worm is transmitted to a worm gear, thus rotating the whole drive sleeve to which the worm gear is fixed.
Once the drive sleeve is rotated, the stem nut also rotates together with the drive sleeve due to a female spline formed in the peripheral inner surface of the drive sleeve engaging with a male spline formed in the peripheral outer surface of the stem nut. When the stem nut rotates, the valve stem is reciprocally moved due to rotation of a valve stem screwed into the stem nut being regulated, thus performing the opening and closing action of the valve.
The stem nut is made from a copper alloy material which is excellent in sliding properties with respect to the valve stem. Moreover, the stem nut is designed so as to be able to be replaced independently without disassembling the electric motor-operated valve drive device as a whole.
A rotary torque from the drive sleeve is transmitted to the stem nut through the spline engagement. On the other hand, thrust from the valve stem is transmitted to the drive sleeve by fixing the stem nut in the axial direction inside the drive sleeve, and is transmitted to the housing through a bearing.
Accordingly, it is necessary to firmly fix the stem nut in the axial direction inside the drive sleeve (for example, see Japanese Un-examined Patent Application Publication No. 2001-193861). More specifically, it is fixed by a stem nut fastening screw abutting to the upper end surface of the stem nut, having a male screw on the outer peripheral surface thereof to be screwed into a female screw provided on the inner peripheral surface of the drive sleeve. The stem nut fastening screw, as illustrated in the drawing, is formed in a two-step structure, in order to prevent loosening.